Magnetic resonance surgery using heat waves produced with a laser fiber

ABSTRACT

Surgery is performed with pulsed heat means that selectively destroys tissue in a region within a patient. The size of the region destroyed is dependent upon the frequency of the pulses of the pulsed heat means and thermal conductivity of the tissue of the patient. The pulsed heat means can be a coherent optical source that is guided by laser fiber to the tissue to be destroyed. In another embodiment the pulsed heat means is a focussed ultrasound transducer that dissipates ultrasonic energy at a focal point within the region of tissue to be destroyed. A magnetic resonance imaging system employing a temperature sensitive pulse sequence creates an image of the tissue and the region being heated to allow a surgeon to alter the position of the pulsed heat means or vary the pulse frequency.

This application is a division, of application Ser. No. 07/751,259, nowU.S. Pat. No. 5,291,890, filed Aug. 29, 1993.

BACKGROUND OF THE INVENTION

The present invention relates to surgery performed by local heatingguided by magnetic resonance (MR) imaging methods of imaging, and moreparticularly to surgery performed by pulsed local heating guided bymagnetic resonance (MR) imaging.

Conventional Magnetic Resonance Imaging (MRI) provides the radiologistwith cross sectional views of the anatomy for diagnosis of pathology.MRI provides excellent contrast between different tissues and is usefulin planning surgical procedures. A tumor is much more visible in an MRimage than as seen in actual surgery because tumor and normal tissuesoften look similar in surgery. The tumor can also be obscured by bloodduring surgery. Researchers at Brigham and Womens Hospital, Boston,Mass. have proposed treatment of deep lying tumors by laser surgery. F.A. Jolesz, A. R. Bleire, P. Jakob, P. W. Ruenzel, K. Huttl, G. J. Jako,"MR Imaging of Laser-Tissue Interactions", Radiology 168:249 (1989).Thus, in the case of brain tumors, the patient is first scanned in anMRI system to locate the tumor and plan a safe trajectory between theentry and target points. This can be accomplished by a MRI deviceemploying fast scan apparatus such as U.S. Pat. No. 4,961,054 GradientCurrent Speed-up Circuit for High-speed NMR Imaging System by John N.Park, Otward M. Mueller, and Peter B. Roemer, issued Oct. 2, 1990, orU.S. Pat. No. 5,017,871 Gradient Current Speed-up Circuit for High-speedNMR Imaging System, by Otward M. Mueller, and Peter B. Roemer, issuedMay 21, 1991 both assigned to the present assignee and herebyincorporated by reference. A small burr hole is drilled in the skull anda hollow needle containing an optical fiber is then inserted into thetumor. The patient is then placed back into the MRI system to view theregion heated by the laser using a temperature sensitive pulse sequence.Temperature Sensitive pulse sequence is described in U.S. Pat. No.4,914,608 In-vivo Method for Determining and Imaging Temperature of anObject/Subject from Diffusion Coefficients Obtained by Nuclear MagneticResonance, Denis LeBihan, Jose Delannoy, and Ronald L. Levin issued Apr.3, 1990 and hereby incorporated by reference. Experiments on animalsshow that a heated zone above a critical temperature destroys tissue.This zone increases in size with time as the heat is applied to reach asteady state or both temperature and heat flow. If the maximumtemperature is limited to 100 deg. C, then the laser heated zone, thearea exceeding a critical temperature causing destruction of tissue,approaches 1 centimeter in diameter. It is difficult to predict theheated zone geometry because the heat flow depends on the profusion ofblood as well as the tissue thermal properties.

Tumors have been selectively destroyed in cancer patients using focussedultrasound heating at the University of Arizona, B. E. Billard, K.Hynynen and P. B. Roemer Effects of Physical Parameters on HighTemperature Ultrasound Hyperthermia Ultrasound in Med. & Biol. Vol. 16,No. 4, pp. 409-420, 1990 hereby incorporated by reference. Billard etal. disclosed that the control of heat was improved by using short laserpulses where the effect of blood perfusion is negligible. However, sincethey did not image the temperature distribution, it was difficult to hitsmall, deep laying targets.

It would be beneficial to be able to accurately localize heat toselectively kill or destroy tumor tissue without damage to surroundinghealthy tissue.

OBJECTS OF THE INVENTION

It is an object of the present invention to selectively destroy tumorsaccurately with a non-invasive procedure employing the use of magneticresonance imaging, and focussed ultrasound.

It is another object of the present invention to selectively destroytumors accurately with a small degree of invasiveness employing the useof magnetic resonance imaging, and a pulsed laser.

SUMMARY OF THE INVENTION

Pulsed heat is used to selectively destroy tumor tissue of a patientwith a minimum amount of surgery. Magnetic resonance (MR) imaging isemployed to provide to a surgeon performing the procedure images of aregion within the patient being heated, such region including the thetumor tissue. A series of fast scan MR images are used to monitor thetemperature with a diffusion sensitive pulse sequence. The pulsed heatis received by the tumor tissue in the form of coherent optical energyproduced by a laser and guided through optical fiber to a hollow needleplaced into the tumor. Another embodiment employs a focussed ultrasoundtransducer as the heat source with the heat concentrated at a focalpoint. The heat is localized by adjusting the frequency of the pulses,since an oscillating point heat source creates a heat wave that decaysexponentially with distance from the source with a decay rate determinedonly by the frequency. The needle or focal point is positioned by amechanical guide under the control of the surgeon.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawing in which:

FIG. 1 is a logarithmic graph of Frequency vs. heat Penetration Depth ofa pulsed heat means according to the present invention.

FIG. 2 is a schematic block diagram of one embodiment of the presentinvention.

FIG. 3 is a partial perspective view of a patient positioned for surgerywithin the bore of the magnets of one embodiment of the presentinvention employing a laser source and fiber optics.

FIG. 4 is a partial view of a patient positioned for surgery within thebore of the magnets of another embodiment of the present inventionemploying a focussed ultrasound source.

DETAILED DESCRIPTION OF THE INVENTION

By employing the present invention, tumor tissue in a patient can beselectively destroyed by localized heating without affecting thesurrounding healthy tissue. A method of selectively heating materialsuch as diamond is disclosed in Thermal Diffusity of IsotopicallyEnriched ¹² C Diamond by T. R. Anthony, W. F. Banholzer, and J. F.Fleischer Phys. Rev. B Vol. 42, No. 2 Jul. 15, 1990 hereby incorporatedby reference. The source of the heat may be either a focussed ultrasoundtransducer or a laser source routed to the tumor tissue through anoptical fiber. The heat is applied to the tumor tissue in a pulsed oroscillating fashion. This oscillation creates a heat wave either at thetip of the optical fiber or at the focus point of the transducer. Thepulsed heat is produced by a source driven in accordance with asinusoidal component and a constant component, and thus variessinusoidally. Although the sinusoidal component of the applied heatwould imply a negative heating or heat withdrawal, the constant heatterm added to the sinusoidal component keeps the heat flow positive.However, the constant heating from a point source steadily adds to thebackground thermal distribution. The temperature distribution T may beexpressed as T(r, t) with r being the radius from the center of thepoint of application, and t being time. The temperature distributionsatisfies the diffusion equation:

    D∇.sup.2 T(r,t)+dT(r,t)/dt=Q(r,t)/ρc          [1]

where ω is an angular frequency,

ρ is the density of the heated tissue,

c is the specific of the heated tissue

and D is the thermal diffusity of the heated tissue.

In the case of a periodic point heat source of amplitude Q₀ at theorigin r=0, the heat flow becomes:

    D∇.sup.2 T(r,t)+dT(r,t)/dt=(Q.sub.0 /ρc)Cos(ωt)δ(r)                           [2]

with frequency f=ω/2π. A radially symmetric solution is of the form:

    T(r,t)=Q/ρc Exp[-kr]cos(ωt-kr)/r                 [3]

where k=Sqrt [ω/2D], and D is the thermal diffusity. The wavelengthL=2π/k of heat waves depends on the thermal diffusivity D and frequencyf, so that

    T(r,t)=Q/ρc Exp[-kr]cos(2πft-kr)/r                  [4]

The heat from an oscillating point source decays exponentially with acharacteristic distance 1/k as shown in H. S. Carlsow and J. C. JaegerConduction of Heat in Solids 2nd Edition, Oxford, Clarendon Press, 1959at pages 64-70 hereby incorporated by reference. The heat decay is givenby: ##EQU1##

The frequency of the oscillating point source is controlled to vary thesize of the heated region. The size of the heated region can be seenwith the use of a Magnetic Resonant (MR) imaging system employing atemperature sensitive MR pulse sequence. The MR imaging system alsocreates an image of the tissue intended to be destroyed. By varying thefrequency of the oscillating point source, the surgeon can selectivelydestroy a small region of tissue, thus performing non-invasive microsurgery. The operator of the apparatus, such as a surgeon, can adjustthe placement of the oscillating point source and the size of tissuedestroyed while monitoring the images of the heated tissue and thetumor. In an alternative embodiment, the frequency of the oscillatingsource and the placement of the oscillating point source of heat areunder the control of a mechanical guide responsive to the MR temperaturesensitive fast scan imaging system.

Consider heat applied at a point by either a laser fiber or focussedultrasound transducer. The heat may be applied over a spot of up to 1mm. in radius because of the laser optical absorption or diffractionlimit of focussed ultrasound. The thermal diffusivity D of tissue issimilar to that of water, which is 0.0015 cm² /sec. The penetrationdepth for a given frequency f is tabulated below

                  TABLE 1                                                         ______________________________________                                        FREQUENCY (Hz) PENETRATION DEPTH (mm)                                         ______________________________________                                        0.001          6.9                                                            0.01           2.2                                                            0.1            0.69                                                           1              0.22                                                           10             .069                                                           ______________________________________                                    

The temperature profile decays exponentially with distance from theoscillating heat source with a penetration depth that, as evident fromTable 1, depends on the frequency. FIG. 1 is a logerithmic graph ofFrequency vs. Penetration Depth.

A schematic block diagram of the magnetic resonance surgery system isshown in FIG. 2. A magnet 10 provides a uniform field for nuclearmagnetic resonance imaging using both gradient coils 20 andradiofrequency (RF) coils 30 to detect the resonance of protons in thepatient. A pulse sequence is applied to the coils by gradient amplifier40 and RF power source 50 to the coils to acquire temperature sensitiveimages rapidly during surgery. Operator's console 60 is used to controlthe imaging. A mechanical guide 70 positions the laser fiber orultrasound transducer 80. Raw data is sent from receiver 90 to asurgical planning and control workstation 100 that displays images 110to the surgeon and enables him to guide the heat source by means of athree-dimensional (3D) pointing device 120 such as a track ball or amouse.

As shown in FIG. 3, a patient 200 lies on a table 210 that moves intothe bore of a two part magnet 260, 270. A laser fiber 230 is insertedinto the patient with a hollow needle 240 guided by a mechanicalpositioning device 250 such as a hydraulic positioner. The trajectory iscomputed from a set of images of the patient taken during surgeryplanning. A safe trajectory from the entry point to the target does notintersect critical anatomy such as large blood vessels. Heat is appliedto tumor tissue 280 by periodically pulsing the laser through laserfiber 230 (i.e., a fiber optic material) to selectively destroy tumor280 while the operator views a temperature sensitive magnetic resonanceimage. More than one needle may be required to remove an irregularshaped tumor.

An alternative embodiment (not shown) may employ a heat source thatcreates heat over a line segment instead of a point.

As shown in FIG. 4, patient 200 is placed on a table 310 designed toaccommodate a focussed ultrasound transducer 330 in a water bath 320.The ultrasound transducer 330 can be moved inside the bore of magnets260, 270 to focus on different locations within patient 200. Ultrasoundtransducer 330 is focussed onto the tumor tissue 280, avoiding bone orair in the path of the ultrasound beam, and pulsed to selectively heattumor tissue 280 at the focal point of the ultrasound transducer. Theultrasound transducer is moved while the surgeon views cross sectionaltemperature sensitive images.

While several presently preferred embodiments of the invention have beendescribed in detail herein, many modifications and variations will nowbecome apparent to those skilled in the art. It is, therefore, to beunderstood that the appended claims are intended to cover all suchmodifications and variations as fall within the true spirit of theinvention.

What is claimed is:
 1. A magnetic resonance (MR) surgery system thatcomprises:a) an invasive device having a tip adapted for being insertedinto a patient, the invasive device capable of transmitting opticalenergy at an application point proximate to the tip; b) an opticalsource for applying heat waves by pulsing optical energy through the tipof the invasive device at a frequency f and amplitude Q at theapplication point to create a heated region having a temperaturedistribution T(r,t) around the application point described by:

    T(r,t)=Q/ρc Exp[-kr]cos(2πft-kr)/r                  [4]

where Q is the amplitude of heat provided; r is the radius from thecenter of the heated region; t is time; ρ is density of the heatedregion; c is specific heat of the heated region; and k=Sqrt (ω/2D),where D is the thermal diffusity of the tissue in the heated region; c)MR compatible positioning means, connected to the invasive device, forpositioning the tip of the invasive device such that the applicationpoint is positioned to cause a selected tissue within the patient tohave the desired temperature distribution T(r,t); d) an MR imaging meansadapted for creating fast scan MR images of the temperature distributionT(r,t) around the application point during surgery using a temperaturesensitive pulse sequence; and e) display means, coupled to the MRimaging means, for interactively displaying the temperature sensitiveimages to an operator to allow the operator to control the temperaturedistribution T(r,t).
 2. The MR surgery system of claim 1 wherein theoptical source comprises a laser.
 3. A method of performing heat surgeryon a patient, as guided by a magnetic resonance (MR) imaging apparatuscapable of producing temperature sensitive MR images, comprising thesteps of;a) determining a position of a selected tissue to be destroyedin said patient with said MR imaging apparatus, b) positioning a tip ofan optical fiber at an application point near the selected tissue withinsaid patient; c) determining a desired temperature distribution T(r,t);d) pulsing optical energy at a frequency f and amplitude Q in order tocreate a heated region having the desired temperature distributionaround the application point, such that:

    T(r,t)=Q/ρc Exp[-kr]cos(2πft-kr)/r                  [4]

where Q is the amplitude of heat provided; r is the radius from thecenter of the heated region; t is time; ρ is density of the heatedregion; c is specific heat of the heated region; and k=Sqrt (ω/2D),where D is the thermal diffusity of the tissue in the heated region; e)monitoring the temperature distribution T(r,t) with said MR imagingapparatus; and f) adjusting the frequency f, the amplitude Q and thelocation of the application point.